Nanoporous dielectric materials are gaining prominence in the recent years as they are finding applications in a wide range of fields including photonics, catalysis, semiconductor processing, biosensors and bioimaging. For example, because of their extremely low refractive index, these materials have been considered as a better and a cheaper alternative to Teflon AF in liquid core waveguide applications. In addition, the relatively large surface area associated with these materials could be efficiently utilized to serve as high density substrates for biomolecule immobilization. With decreasing feature sizes, new materials with ultra low dielectric constant are becoming an increasingly important requirement in the semiconductor industry at present to replace conventional silicon dioxide as the interconnect insulation material. Suitable materials with ultra low dielectric constant have to be obtained in order to minimize the RC interconnect delays.
Various methods have been proposed for the preparation of nanoporous dielectrics. Among the more common are the surfactant templating method for ordered porous structures (see, e.g., Y. Lu, R. Ganguli, C. A. Drewien, M. T. Anderson, C. J. Brinker, W. Gong, Y. Guo, H. Soyez, B. Dunn, M. H. Huang, and J. I. Zink, “Continuous formation of supported cubic and hexagonal mesoporous films by sol-gel dip-coating,” Nature, vol. 389, pp. 364-368, 1997 and C. J. Brinker, Y. Lu, A. Sellinger, and H. Fan, “Evaporation-Induced Self-Assembly: Nanostructures Made Easy,” Advanced Materials, vol. 11, pp. 579-585, 1999) and the porogen extraction method for random pore structures (see, e.g., B. Lee, Y.-H. Park, Y.-T. Hwang, W. Oh, J. Yoon, and M. Ree, “Ultralow-k nanoporous organosilicate dielectric films imprinted with dendritic spheres,” Nat Mater, vol. 4, pp. 147-150, 2005 and M. Ree, J. Yoon, and K. Heo, “Imprinting well-controlled closed-nanopores in spin-on polymeric dielectric thin films,” Journal of Materials Chemistry, vol. 16, pp. 685-697, 2006). In each of these methods, nanoporosity is introduced by forming a nanocomposite film of a thermally labile species (porogen) within an otherwise monolithic matrix material, followed by a high temperature heating step. Calcination of the porogen leaves behind nanopores in the monolithic matrix material thereby effectively decreasing the dielectric constant and refractive index of the film.
The formation of porous films by conventional porogen or surfactant templating approaches typically requires highly-controlled slow-curing processes to prevent pore collapse. For example, temperature must be closely controlled during heating, at curing, and then heating to volatilization. The formed films may suffer from large residual stresses during the cooling run which may initiate buckling and cracking in the films especially when thick films are desired for waveguide applications. See, e.g., W. Oh, T. J. Shin, M. Ree, M. Y. Jin, and K. Char, “Residual Stress Behavior in Methylsilsesquioxane-Based Dielectric Thin Films,” Molecular Crystals and Liquid Crystals, vol. 371, pp. 397-402, 2001 and W. Oh and M. Ree, “Anisotropic Thermal Expansion Behavior of Thin Films of Polymethylsilsesquioxane, a Spin-on-Glass Dielectric for High-Performance Integrated Circuits,” Langmuir, vol. 20, pp. 6932-6939, 2004. The versatility of these materials coupled with growing demand is driving researchers to rethink their fabrication methodology to achieve them in the most energy efficient and commercially attractive way.
Another technique of formation of nanoporous films is based on the deposition of nanoparticles through gas evaporation techniques. See, e.g., S. Nozaki, H. Ono, K. Uchida, H. Morisaki, N. Ito, and M. Yoshimaru, in Interconnect Technology Conference, 2002. Proceedings of the IEEE 2002 International, (2002).
Ultra large surface area (201 m2/g) films have previously been reported. See, e.g., T. Miki, K. Nishizawa, K. Suzuki, and K. Kato, “Preparation of nanoporous TiO2 film with large surface area using aqueous sol with trehalose,” Materials Letters, vol. 58, pp. 2751-2753, 2004 and M. R. Mohammadi, M. C. Cordero-Cabrera, D. J. Fray, and M. Ghorbani, “Preparation of high surface area titania (TiO2) films and powders using particulate sol-gel route aided by polymeric fugitive agents,” Sensors and Actuators B: Chemical, vol. 120, pp. 86-95, 2006. The surfaces of these ultra large surface area films, however, tend to be relatively hydrophilic and relatively rough.
Ultra large surface areas have also been reported for porous carbon based materials. These materials, however, are generally not transparent or smooth.
Surface area values for silica aerogels have been reported to be 750-1100 m2/g. See, e.g., B. Zhou, J. Shen, Y. Wu, G. Wu, and X. Ni, “Hydrophobic silica aerogels derived from polyethoxydisiloxane and perfluoroalkylsilane,” Materials Science and Engineering: C, vol. 27, pp. 1291-1294, 2007 and L. L. Aranda, “Silica aerogel,” Potentials, IEEE, vol. 20, pp. 12-15, 2001. Preparation of these aerogels, however, typically requires controlled supercritical drying, etc. Also, silica based aerogels tend to be relatively hydrophilic which results in a moisture absorption which may, in turn, lead to deterioration of the material. Post treatment is thus typically required to render these materials hydrophobic to minimize moisture absorption.
The formation of porous films by conventional porogen or surfactant templating approaches typically requires highly-controlled slow-curing processes to prevent pore collapse. The formed films may suffer from large residual stresses during the cooling run which may initiate buckling and cracking in the films. See, e.g., W. Oh, T. J. Shin, M. Ree, M. Y. Jin, and K. Char, “Residual Stress Behavior in Methylsilsesquioxane-Based Dielectric Thin Films,” Molecular Crystals and Liquid Crystals, vol. 371, pp. 397-402, 2001 and W. Oh and M. Ree, “Anisotropic Thermal Expansion Behavior of Thin Films of Polymethylsilsesquioxane, a Spin-on-Glass Dielectric for High-Performance Integrated Circuits,” Langmuir, vol. 20, pp. 6932-6939, 2004.
To achieve seamless integration of nanoporous thin films in a miniaturized format, it is important to be able to generate microscale patterns of these materials, while preserving their unique properties. For example, high density microarrays of such films can enable massive multiplexing and low cost fabrication per chip for the development of biological assays and sensors. Significant cost reduction can be achieved through simpler patterning processes for extant back end integration schemes currently practiced in the semiconductor industry.
Patterning nanoporous thin films presents a formidable challenge. The high surface areas typically associated with these materials together with accessible porosities, especially for films with porosities >50%, precludes the use of standard lithographic techniques. Standard photolithography followed by wet chemical/plasma etching significantly damages these surfaces and causes loss of functionality, thereby requiring post repair of their damaged structures. Kim et. al., “Photopatterned Nanoporous Media,” Nano Lett., vol. 4, pp. 1169-1174 (2004), report a direct photo-patterning technique for nanoporous dielectrics through the use of a photosensitive acid generator, whereby selective crosslinking of PMSSQ can be achieved through UV exposure via a photomask.